A very significant limitation on the operation and reliability of electronic components is the efficient extraction of heat. Unless heat generating components are provided with an efficient heat transfer mechanism to maintain them within a predetermined operating temperature range, the power and useful life of the components are severely limited. Excessive over-heating of electronic components may cause their destruction.
The problem of heat removal is increased for power supply modules having components of different heights, and which are densely populated on a substrate. Particularly, with integrated on-board power supplies (IOPs), such as DC-to-DC converters the problem of heat removal is acute, since power supplies generate a substantial amount of heat. An IOP is a module which is self-contained and typically mounted on a circuit board containing the logic components of the computer system. In order to minimize the total size of the system, the spacing or pitch between the various circuit boards of the computer system which are inserted in parallel fashion in the back plane of the computer system is kept to a minimum. Therefore, the height or profile of the IOP, including the heat generating components is kept, for example, less than 0.3 inches, and the height of the IOP including the heat dissipating components, such as a heatsink, must be kept less than the inter-board spacing.
A typical IOP includes heat generating power train components, for example, transformers, field effect transistors (FETs), rectifiers, and chokes. In general, the power train components are of varying surface topologies and of different heights. The components are provided with leads to permit attachment to a substrate. The leads are inserted in plated through holes or vias and electrically connected by solder to conductive circuit traces formed on the substrate.
It is a common approach in the cooling of heat generating components having different heights to provide the components, with poli-directional, or mono-directional heatsinks. Typically, the heatsinks are individually attached to the tops of the components for the purpose of dissipating heat. The drawbacks of such an approach is that the individual heatsinks are of necessity limited in their footprints, must each be sized to conform to the geometry of the components being cooled, must be aligned carefully with the components and are otherwise limited by the overall size of the system into which the module is incorporated. It is tedious and expensive to adapt individually heatsinks to discrete heat generating components 11-14 which are diverse in placement, shape, and height.
Alternatively, the power train components of the IOP are cooled by a single heatsink which is placed in thermal contact with the opposite side of the substrate on which the components are located. Although this scheme is fairly simple to assemble, it is less efficient in drawing heat from the components through the substrate, particularly if the components are surface mounted and lack leads protruding through the substrate for conducting heat, or if the substrate is made of relatively inexpensive material, for example, epoxy-glass, which has poor thermal conduction characteristics. Moreover, because the IOP is usually mounted on a circuit board, an undesirable cut-out may be required in the circuit board to accommodate the heatsink. Furthermore, some power train components generate so much heat that direct thermal contact with a heatsink is required to maintain the components at a desirable temperature.
Another approach is to provide a single heatsink for the group of components. The main drawback of this approach is that it is a complex task to thermally mate the rigid heatsinks with the components. Generally, a thermally conductive epoxy is used to cover the components in order to create a relatively smooth surface to mate with the heatsink. Thermally conductive epoxies require special handling, long curing time, and generally increase the manufacturing cost of the module. Also, the thermal conductivity of epoxies is relatively poor, typically about 10% than that of metals, requiring that the module be operated at substantially reduced power.
Alternatively, the mating surface of the single heatsink is especially machined to conform to the external mating surfaces of the components. This approach requires additional labor and careful alignment of the heatsink and components during assembly.
Accordingly, the known solutions for cooling an IOP increase the cost of assembly, and do not always provide for effective cooling of discrete and variously shaped heat generating components.
Therefore, it is desirable to provide an apparatus and method for reliably cooling an integrated on-board power supply which is simple to assemble, uses readily available inexpensive materials, and is easily adaptable to mass production methods at reduced costs.